(1) Field of the Invention
The present invention relates to a culvert joint which can follow an expansion and contraction of distance or an uneven subsidence between the opposing culvert sections, more specifically to a culvert joint designed to regulate the distance between its strengthening members which bear the earth pressure.
(2) Description of the Prior Art
In the conventional joint for concrete culvert, an elastic material like rubber or synthetic resin is employed to constitute the junction of the culvert; the two ends of a short tubular flexible member are anchored around the inside surfaces of opposing culvert sections or the flexible member is bolted at the rear to suspend a part of said member, thereby minimizing the distortion in the flexible member due to difference between internal and external water pressure in the culvert. In both cases, the earth pressure acting at the junction of culvert sections is borne by a concrete mass poured behind the whole flexible member so that the flexible member simply bears the internal water pressure or the underground water pressure.
Thus in the event of a heavy uneven subsidence of the ground or displacement of the culvert sections in a longitudinal direction or in a direction perpendicular to the longitudinal direction during an earthquake, with result that the gap at the junction remarkably widens, the mud and sand located around the culvert come into direct contact with the flexible member, causing a heavy earth pressure to act directly on the flexible member, whereupon a harmful distortion develops in the flexible member and not only is the water flow impeded, but also the flexible member loses its durability or is broken very often.
To eliminate these troubles, various measures are taken. For instance, anchor members which have a seat for fitting the flexible member and cavities at the inside and outside of the seat are respectively provided at the opposed open ends of culvert sections to be joined together. A short tubular flexible member made of rubber or synthetic resin to smoothly absorb a heavy uneven subsidence in a state of maintaining watertightness of joint block is provided extending over the seats of the opposed anchor members, while a large number of strengthening members to withstand the water pressure and earth pressure at the internal and external positions of the flexible member and transmit these forces to said anchor members, are provided in such an arrangement that both ends of the strengthening members are held displaceable to a certain extent within the cavities of said anchor members. Thus said short tubular flexible member is wholly protected, thereby preventing it from being damaged from inside and outside by rolling stones, mud and sand, wood, iron piece or water.
In a joint having such a strengthening member, however, numerous parallel gaps must be left between the numerous strengthening members so that the displacement in a direction perpendicular to the longitudinal direction of culvert due to an uneven subsidence of adjacent culvert sections can be smoothly absorbed. Thereby said gap between the strengthening members should be appropriately maintained without moving to one side so that mud and sand around the outside of the joint blocks may not enter to the flexible member through the gaps between the strengthening members.
It is conceivable to provide a padding at the gap for the purpose of preventing entry of mud and sand through the gap, but such a padding must be a soft, elastic material like sponge rubber with a relatively low resilience or a filler such as a plastic one like asphalt or putty, so that the resistance to deformation of the junction under progress of uneven subsidence can be minimized. However, the low resilience and plastic deformability of the fillers are likely in the junction of underground culvert to result in that with progress of uneven subsidence the mud and sand around the junction causes the distances between the strengthening members to become uneven and in consequence the strengthening members to move to one side, thereby developing a gap between them which permits penetration of mud and sand and consequent decrease of durability or damage of the flexible member as mentioned before. Thus even when a padding is provided between the strengthening members, the regulation of their distance is important.
The penetration of mud and sand through the gap caused by moved strengthening members restrains a free displacement of the flexible member which is required to follow the uneven subsidence of the culvert, thereby causing damage to the flexible member; when water is flowing in the culvert, the resistance to water flow increases owing to the smoothness of internal surface at the junction being impeded, leading to a drop in the joint performance and in the joint durability.